The present invention relates to digital multi-level multi-phase modulation communication systems.
Whereas digital multi-phase PSK modulation communication systems have been in practical use, multi-phase multi-level modulation communication systems have recently come into use to enhance the efficiency of the use of frequency bands. Among these systems, the 16 QAM communication system is most popular.
In such communication systems, differential code conversion is usually achieved so that a demodulator section requires no absolute phase of the reproduced carrier wave. However, this differential code conversion requires differential encoder circuits on the transmitting side and differential decoder circuits on the receiving side, thereby increasing constituent elements with an increase in the phase and level of the carrier wave. This method further has an inherent shortcoming that a single bit error on the transmission path is enlarged into two bit errors on the receiving side.
For details of the above described 16 QAM communication system, reference is made to:
(1) Izumi Horikawa, "Characteristics of a High Capacity 16 QAM Digital Radio System on a Multipath Fading Channel", ICC '79 Conference Record, pp. 48.4.1-48.4.6, June 10-14, 1979, and PA1 (2) Japanese Patent Application Disclosure No. 109811/77 (wherein the technique of Reference 1 is disclosed).